Over 170 million people worldwide are chronically infected with hepatitis C virus (HCV), placing them at risk for liver failure and liver cancer. The current treatment for HCV consists of pegylated interferon alpha (IFN-a) with ribavirin, yet this treatment is only effective in about half of all patients. Therapeutic options have improved with the recently approved protease inhibitors, yet cure rates remain less than ideal. Additionally, protease inhibitors add to already substantial side effects, and viral resistance is rapidly becoming a concern. As the updated standard of care still relies on IFN-based therapy, an understanding of the antiviral state induced by IFN treatment is crucial for improvements in therapeutic developments. The mechanisms behind IFN-induced antiviral actions are not fully understood, particularly in regards to which interferon-stimulated genes (ISGs) have antiviral effector functions. We are interested in identifying ISGs with the ability to inhibit HCV infection with the long-term goal of discovering novel targets for more directed and successful anti-HCV therapies. We have identified the interferon-induced transmembrane protein (IFITM) family of molecules as having inhibitory actions on HCV in cell culture systems. IFITM1 alters the association of a cellular receptor for HCV, CD81, with other HCV coreceptors and components of the hepatic tight junction. Our next goal is to characterize the biological functions of these molecules, particularly that of IFITM1, in the context of HCV infection and to determine the mechanism by which IFITM1. We propose to accomplish these goals through the following aims: (1) Determine the stage(s) of the HCV replication cycle affected by IFITM expression and (2) Identify the molecular function by which IFITM1 regulates CD81-dependent complexes. Together, the experiments proposed will describe the function of a set of IFN-induced effector molecules and identify their role in the antiviral state established following IFN treatment. These studies will provide a more complete understanding as to the mechanism of how successful IFN treatment inhibits HCV to allow for more targeted therapeutic approaches.